Process of recovering unpolymerized photopolymer from printing plates

ABSTRACT

A photopolymerizable composition of matter useful for photopolymeric printing surfaces is disclosed. Printing surfaces prepared from the composition are strong and wear well, resist solvents and abrasives, and are suitable for intaglio or gravure printing.

This is a division of application Ser. No. 093,622, filed Nov. 13, 1979now U.S. Pat. No. 4,269,930.

BACKGROUND OF THE INVENTION

Photopolymeric printing plates are widely used in the printing industrywhere they have many advantages but suffer from the well recognizeddisadvantages of being relatively soft and susceptible to attack by manyorganic solvents. These disadvantages make conventional photopolymericprinting surfaces unsuited for many types of printing involvingmechanical stress, surface wear, or solvents or abrasive materials ininks. This particularly includes intaglio or gravure printing thatinvolves doctor blades wearing against the print surface, strongsolvents in the inks, high speeds, and long runs. Gravure has remainedbeyond the capacity of photopolymers and has required more expensivemetal surfaces.

I have discovered a photopolymerizable composition of matter thatproduces a harder, tougher, and more wear resistant photopolymer thanpreviously available. The composition is useful in producing harderprinting surfaces suitable for types of printing that photopolymerscould not previously withstand, particularly intaglio or gravureprinting.

SUMMARY OF THE INVENTION

The photopolymerizable composition of matter of my invention comprises amixture of:

a. from about 70-95%, by weight, of an alcohol-soluble linear polyamide;

b. from about 0.5-15%, by weight, of a low molecular weight dicarboxylicacid diester; and

c. an effective amount of at least one photosensitizer and at least onepolymerization inhibitor.

In preparing a photopolymeric printing surface according to myinvention, I coat a backing or base with a mixture of alcohol and myphotopolymerizable composition. The alcohol serves as a solvent andcarrier allowing the mixture to be coated evenly; and then the alcoholevaporates, leaving a dry and uniform coating of the photopolymerizablecomposition.

Actinic light passing through a photographic image photopolymerizes aportion of the coating; and an etching or washing fluid removes theunexposed portion of the coating, leaving an etched printing surface.The etched surface may, optionally, be coated with a thin layer of afluorotelomer material to provide a smooth, low friction surface finishwith enhanced wear resistance and ink release properties. The etchedpolymer, whether coated or uncoated, may also, optionally, be baked toincrease its hardness.

Printing surfaces prepared according to my invention are tougher,harder, and more durable than photopolymeric materials currentlyavailable to the printing industry. This, together with their resistanceto most organic solvents, makes them useful in printing applicationsbeyond the capacity of previous photopolymers. Other uses and advantagesof my invention may occur as knowledge about it spreads.

BRIEF DESCRIPTION OF THE FIGURE

The FIGURE is a schematic diagram of preferred steps in my process forpreparing a photopolymeric printing surface from my photopolymerizablecomposition.

DETAILED DESCRIPTION

The photopolymerizable composition of matter of my invention comprises,in its simplest form, a mixture of:

a. from about 70-95%, by weight, of an alcohol-soluble linear polyamide;

b. from about 0.5-15%, by weight, of a low molecular weight dicarboxylicacid diester; and

c. an effective amount of at least one photosensitizer and at least onepolymerization inhibitor.

The polyamide component of this composition may be any of the polyamidesthat are conventionally used in photopolymeric compositions. Theseinclude copolyamides that are soluble in common solvents or mixtures ofsolvents, such as lower aliphatic alcohols or mixtures of these alcoholswith water, ketones, or aromatic compounds. Typically, copolyamides areprepared by polycondensation or activated anionic polymerization of twoor more lactams having from five to thirteen ring members, or bypolycondensation of the appropriate dicarboxylic acids and diamines, asis well known to those skilled in the art. Particularly suitable linearpolyamides are those described in U.S. Pat. Nos. 3,512,971; 3,516,828;and 4,144,073, the disclosures of which are hereby incorporated byreference.

By the term "low molecular weight dicarboxylic acid diester" is meantsaturated and unsaturated dicarboxylic acid diesters having up to eightcarbon atoms in the acid backbone and up to about four carbon atoms inthe alcohol moiety appended at the ester linkage. Preferred diestersinclude diethyl malonate, diethyl fumarate, diethyl sebacate, dipropyladipate, and diethyl maleate. There is some evidence, although presentlyinconclusive, that the resistance of the photopolymer of my invention tocertain strong organic solvents, such as methyl ethyl ketone, decreaseswith increasing molecular weight of the acid portion of the diestercomponent. Hence, the most preferred diester components of myphotopolymerizable composition are those having short acid backbones,i.e., diethyl malonate, diethyl fumarate, and diethyl maleate.

The photosensitizers suitable for use in my composition are thosecompounds that decompose to radicals upon exposure to light orirradiation and initiate polymerization. Such compounds are well knownto those skilled in the art and include acyloins and their derivatives,such as benzoin, benzoin alkyl ethers, alpha-methylolbenzoin and itsethers, alpha-methylbenzoin, diketones and their derivatives,monoketals, and substituted and unsubstituted quinones, such asanthraquinone. Many other photosensitizers are available and probablyworkable, including inorganic materials such as potassium dichromate,which I have used successfully.

It is possible to use a single photosensitizer or mixtures ofphotosensitizers in my composition. Generally, the photosensitizer isincluded in the photopolymerizable composition in amounts ranging from0.01-5% by weight. Of course, the precise amount required will dependupon the particular polyamide in the composition, as well as theparameters of the photopolymerization process. Those skilled in the artwill have little difficulty in selecting a particular photosensitizer ormixture of photosensitizers for a specific application.

Likewise, the polymerization inhibitors included in my composition areany of the conventional inhibitors for preventing thermalpolymerization, such as methylene blue, hydroquinone, p-methoxyphenol,etc. These compounds are incorporated into my composition individuallyor as mixtures, in amounts ranging from 0.01-2% by weight. Again, theprecise amount will depend upon numerous factors and, in particular, thenature of the process used to photopolymerize the composition.

Other components not necessary to obtain the goal of my invention may beincluded in my photopolymerizable composition. These include, forexample, unsaturated monomers, such as acrylamides, as well as dyes andpigments. Any added materials should withstand solvents and temperaturesto be encountered later in the life of the polymer.

To prepare the composition of my invention, the various components inthe appropriate quantities are dissolved in a lower aliphatic alcohol,preferably n-propanol, to form a mixture having a consistency suitablefor spraying or otherwise coating my composition on a flat orcylindrical base.

Alternately, the composition can be prepared by stripping unexposedphotopolymeric material from commercial printing plates containingalcohol-soluble linear polyamides, photosensitizers, polymerizationinhibitors, and a low molecular weight dicarboxylic acid or anhydride,in general accordance with U.S. Pat. Nos. 3,512,971 or 3,516,828. Thisis done with a stripping fluid formed as a mixture of water and a loweraliphatic alcohol, e.g., ethanol, as generally known. Additionalphotosensitizers and inhibitors may be added to the strippedphotopolymeric composition to adjust its exposure sensitivity. Theresulting mixture of stripping fluid and stripped photopolymericcomposition is then heated to cause phase separation.

While not intending to be bound by theory, I believe that this heatingof the mixture causes the dicarboxylic acid or anhydride to convert tothe corresponding dicarboxylic acid diester. The aqueous phasecontaining water, alcohol, and low molecular weight monomers isdiscarded; and the organic phase, which contains the photopolymerizablecomposition, is dissolved in alcohol for coating on a base plate orcylinder.

Further details of the preparation of photopolymerizable compositionsaccording to my invention appear in the examples following this detaileddescription.

My photopolymerizable composition can be used to form many types ofphotopolymeric printing surfaces. These include both relief surfaceshaving an ink-bearing level raised from surrounding areas and intaglioor gravure surfaces having ink-bearing regions formed as shallowrecesses. A doctor blade generally wipes excess ink from intagliosurfaces, and both intaglio and relief surfaces can be used either indirect or offset printing. Variations on these processes are extensiveand generally known; and there is reason to believe that myphotopolymeric composition will succeed at most of them, especiallysince it is hard, tough, and wears well against a doctor blade.

The photoresponsiveness of my composition is similar to commerciallyavailable photopolymer printing plates made according to U.S. Pat. Nos.3,512,971 and 3,516,828. It resolves fine detail and images and etchescleanly and accurately so that it can be used for many types ofprinting. The processes for imaging and etching my material aregenerally similar to known processes for imaging and etching prior artphotopolymeric plates. These processes expand somewhat to apply mymaterial to more uses, however, because its increased hardness and itsability to withstand extensive running against a doctor blade make ituseful for both flat and cylindrical gravure printing, either direct oroffset. The gravure image area can be divided into a regular array ofshallow cells in a generally known way or can be formed of individuallyshaped fine lines in the manner of a handmade steel engraving plate. Forthis, a conveniently large drawing can be photographically reduced andimaged on my material for printing precious documents with either flator rotary surfaces. So my material succeeds in the same printingapplications as previous photopolymers and wears longer because of itsgreater strength and hardness, and these characteristics also give it awider range of uses that previous photopolymers could not attain.

My photopolymerizable composition is coated on a support or basematerial that is suitable for the printing circumstances. This includesboth flat plates and cylinders formed of a wide variety of materials,including metals and plastics. The coating can be applied in many waysas known in the coating arts. The general requirements are that thesupport or base functions satisfactorily for the printing involved, thatthe polymer coating securely bonds to the base, and that the bond is notattacked or weakened by solvents or temperatures to be encounteredlater. These limitations allow many variations, however; some of whichare described below.

My composition photopolymerizes on exposure to actinic radiation, whichin this case is ultraviolet light as already developed for exposing andimaging photopolymers and photoresist materials. Coating and otheroperations that occur before exposure are accomplished in yellow light.

In making a photopolymeric printing element according to my invention, Ifirst prepare the surface of a support base to receive a coating of mypolymerizable composition. Although this can vary with differentmaterials and configurations, my experience has been with metal surfacesthat I polish and then cover with a thin and even primer coating. Asuitable primer must resist baking temperatures and alcohols and othersolvents that will later be encountered, and it generally must provide asecure and durable bond between the metal base and the polymer coating.I have succeeded with both an industrial primer enamel and an acrylicprimer, and there is reason to believe that many other primers willwork.

I spray the primer coating evenly over the metal surface using either anair or airless spray gun and then bake the primer coating dry. A primercan also be applied with other spraying and coating techniques with thegoal being a smooth and even coating having no entrapped gas to causeimperfections. The primer coating is preferably very thin and no thickerthan necessary to achieve a uniform coat.

After the primer coating has dried, I sand or grind it with a very finegrit to remove surface gloss and leave a mat finish. It may also bepossible to prepare a satisfactory mat surface for receiving mypolymerizable coating directly on a metal surface by using a "pickling"composition, sandblasting, or other metal surfacing techniques. There isreason to believe that my polymerizable composition can be coated onexisting gravure cylinders having metal surfaces of copper, brass,nickel, and other metals, either by using a suitable primer coating or asurface treatment. Also, plastic surfaces may be prepared in differentways to receive my polymerizable coating. The alternatives allow mypolymer to be used on a wider variety of bases than are practical formetallic surfaced gravure cylinders.

I next coat my photopolymerizable composition onto the prepared surfaceof the base by dissolving my composition in a lower aliphatic alcohol,preferably n-propanol to make it sufficiently fluid. The alcohol acts asa diluent or carrier enabling the composition to be coated evenly overthe entire surface of the base. This can be done by using any of severalknown coating techniques, and I have succeeded by spraying the mixtureonto the base surface. The preferred thickness of the coating depends onthe type of printing to be done, and I have coated my photopolymerizablecomposition on gravure cylinders at a rate of about 1 milliliter persquare inch. Letter press printing would require a much thicker coating,which could be built up in even layers to any desired thickness.

After coating my photopolymerizable composition evenly on the base, Iheat the coating to a temperature of about 150° F. to evaporate thealcohol, leaving only the photopolymerizable composition as a drycoating. The surface of the coating should be glossy smooth; and toachieve this, I polish the dry surface after heating. Coating techniquesthat directly produce an adequately smooth surface could eliminate thepolishing step and would require only drying the coating withoutnecessarily heating.

My photopolymerizable coating is then ready for imaging and etching bythe same general methods already in use in the art. Negative or positiveimages placed in close contact with the smooth surface of my coatingallow actinic ultraviolet light to polymerize portions of the surface inthe pattern of the image used. Then a washing or etching fluid formed asa mixture of water and a lower aliphatic alcohol, such as ethanol,removes unpolymerized material leaving polymerized material forming aprinting surface in the desired image.

A thin residue of dilute polymerizable material remains from the etchingprocess. To remove this, the etched surface is postexposed to actinicultraviolet light for an exposure time that is ordinarily aboutone-fifth the length of the imaging exposure for further polymerizingand hardening the polymer surface without polymerizing the diluteresidue. Then the surface can be cleaned with an alcohol rinse; and Iprefer n-propanol and gentle wiping, leaving a glossy and residue-freesurface.

At this point, my etched photopolymer resists solvents and is harderthan known photopolymers so that it can be used for printing. However,it also can be made much harder by baking, which I prefer for improvingits printing capacity. My etched photopolymer can be baked directly athigh temperatures around 500° F., which would destroy conventionalphotopolymeric plates, but which makes my polymer hard, tough, anddurable. The baking does not damage the surface finish of my polymer ordiminish its accuracy or detail; and it hardens and toughens it enoughto withstand the rigors of doctor blades, strong solvents, heavy stress,and long wear.

Before baking, I prefer coating intaglio type photopolymeric surfaceswith a thin layer of a fluorotelomer that provides a smooth surfacefinish with a low friction for wearing well against the doctor blade. Afluorotelomer material that I have found to be particularly effective ismarketed by DuPont under the brand name, Vydax 550, as a dispersion of awhite, waxy, short-chain telomer of a tetrafluoroethylene in atrichlorotrifluoroethane solvent. It provides a low coefficient offriction and high lubricity on the printing surface and affordsexcellent release or anti-stick properties.

My preferred baking step, either with or without a fluorotelomercoating, can be done directly in a conventional oven at temperatures offrom 350° to 550° F. and preferably at 450° to 500° F. Highertemperatures generally produce harder surfaces and increasedbrittleness, and lower temperatures produce less hard surfaces that aremore resilient. For gravure printing, I prefer harder surfaces baked ataround 500° F.; but lower temperatures leaving a printing surface moreresilient might work better in cooperation with some types of doctorblades.

Printing surfaces made according to my invention are tough, hard,durable, highly accurate, and versatile so that they are useful whereprevious photopolymers have failed. This includes flat and rotarygravure and the printing of precious documents such as checks, travelerschecks, stock certificates, etc. made from a host of individual linespreviously requiring handmade engravings. Hardened intaglio surfacesmade by my invention can also be used as dyes for making moldedpolymeric relief plates. Gravure surfaces made by my invention have beenhighly successful in printing precious documents and may even succeed inprinting money.

The strength and hardness of my polymeric material suggests otherpossible uses outside the printing arts where hard, durable plastics arerequired. My composition can be polymerized by heat as well as byactinic light, suggesting that photosensitizers could be eliminated andmy composition could be thermally polymerized to form useful articles.

Although evidence is incomplete and I do not wish to be bound by theory,I believe that the hard and strong polymer of my invention results fromthe reaction of the linear polyamide component of my composition withthe diester component to form a cross linked reaction product that isstrong, hard, and tough. The proportion of the diester component that isable to react with the linear polyamide component and form a strong,hard, and tough polymer is generally within 0.5 to 15% by weight of thecomposition. It is possible that the cross linking produced by less than0.5% of the diester component will also increase the strength andtoughness of the polymer, and it is likely that more than 15% of thediester component will form an uneconomical excess of diester in thecomposition.

The following examples illustrate practice of preferred embodiments ofmy invention and are presented to illustrate and explain and not tolimit the scope of my claimed invention.

EXAMPLE 1 Preparation of Photopolymeric Gravure Cylinder Using Materialsfrom Commercial Photopolymeric Plates

I have salvaged unpolymerized photopolymeric material stripped fromcommercial plates sold by BASF Wyandotte Corporation under the brandname, Nyloprint, and used it to make several gravure cylinders coatedwith my composition. The salvaged material was mixed with a strippingfluid formed as a mixture of water and ethanol as recommended by theplate manufacturer. So far as could be determined, these plates weremade according to the disclosures of U.S. Pat. Nos. 3,512,971 and3,516,828 and contained an alcohol-soluble copolyamide (prepared bypolycondensation of hexamethylene diammonium adipate,4,4'-diaminodicyclohexylmethane salt of adipic acid andepsilon-caprolactam), benzoin methyl ether, p-xylylene-bis-acrylamide,hexamethylene-bis-acrylamide, triethylene glycol diacrylate, butanediolmonoacrylate, maleic anhydride, and hydroquinone.

To adjust the responsiveness of this material to ultraviolet light, Iadded one-half gram methylene blue and one-half gram potassiumdichromate to seven gallons of the stripped mixture. I heated theresulting mixture to a temperature between 180°-190° F. where a phaseseparation occurs, the exact phase separation temperature depending uponthe particular portion of components. Also, the phase separationtemperature slightly exceeds the temperature (believed to be 167°-176°F.) that converts the maleic anhydride to diethyl maleate so that thisconversion occurs by the time phase separation occurs.

I then discarded the aqueous phase and recovered the organic phase thatI dissolved in an equal volume of n-propanol. I stored the resultingphotopolymerizable composition in n-propanol in a dark container toinhibit premature photopolymerization.

I mounted a standard aluminum gravure cylinder on a lathe where Ipolished its surface and spray coated it with an acrylic primer made bythe Borden Company under the Krylon brand name. I baked this coating dryand hard at 150° F. and then sanded the primer surface lightly with anextremely fine grit to remove surface gloss. I have also prepared steelgravure cylinders in a similar way by using an industrial primer bakedat 350° F. as recommended by the manufacturer and sanded to removesurface gloss.

I then spray coated my polymerizable composition in n-propanol evenlyover the primer coating by rotating the cylinder on the lathe and slowlyand evenly moving a spray gun along the length of the cylinder. This andother operations before exposure of the coating were done in yellowlight to avoid premature polymerization.

I coated the polymeric coating on the cylinder at approximately 1milliliter per square inch, which is thick enough for gravure printingpurposes. I then heated the coated cylinder to about 150° F. to dry thecoating, which I then polished to a glossy smooth surface finish.

I then used conventional technology to secure a photographic elementtightly around the cylinder and expose the photopolymerizable coating toultraviolet light as the cylinder rotated on the lathe. I used two 40watt ultraviolet lights to provide the actinic light during a 15 to 20minute exposure.

I then removed the photographic element from the cylinder and used aconventional washing or etching fluid comprising a mixture of water andethanol that I sprayed on the cylinder to remove the unexposed andunpolymerized material. I postexposed the etched cylinder to ultravioletlight for about three minutes to complete the photopolymerization of theunetched material. Then I cleaned etching residue from the cylinder witha rinse of n-propanol followed by lightly wiping the cylinder with aclean cloth.

I next spray-coated the photopolymer surface with a thin layer of afluorotelomer material marketed by DuPont under the brand name, Vydax.Then I baked the Vydax coated cylinder in an oven heated to atemperature of about 450°-500° F. to harden and toughen the etchedphotopolymer printing surface.

Subsequent testing indicated that the cylinder withstood strong organicsolvents, such as methyl ethyl ketone, and made satisfactory gravureprinted copies of the image from the photographic element. The cylinderalso survived endurance tests both running in ink against a doctor bladeand actual runs on a printing press.

EXAMPLE 2 Preparation of Photopolymerizable Composition Using DifferentDiesters

Five photopolymerizable compositions were prepared by mixing analcohol-soluble linear polyamide benzoin methyl ether and a dicarboxylicacid diester in the following proportions:

    ______________________________________                                        Sample Polyamide   Benzoin Methyl Ether                                                                         Diester                                     ______________________________________                                        A      0.50009 gm  0.00954 gm     diethyl                                                                       malonate                                                                      20 μl                                    B      0.50096 gm  0.01025 gm     diethyl                                                                       maleate                                                                       20 μl                                    C      0.60088 gm  0.00984 gm     diethyl                                                                       fumarate                                                                      20 μl                                    D      0.60099 gm  0.01001 gm     diethyl                                                                       sebacate                                                                      20 μl                                    E      0.59969 gm  0.01008 gm     dipropyl                                                                      adipate                                                                       20 μl                                    ______________________________________                                    

The mixtures were dissolved in 1.4 milliliter methanol, poured on a flatplate, dried in a dark room, covered with a negative, and exposed toultraviolet light. The unexposed portions of the plate were removed byetching with a mixture of water and ethanol. The etched photopolymercoating was resistant to methyl ethyl ketone. Samples D and E exhibitedsome peeling from the plate.

I claim:
 1. A process for preparing a photopolymerizable composition ofmatter comprising:a. stripping unpolymerized material from an imagewiseexposed and photopolymerized printing plate that contains analcohol-soluble linear polyamide, photosensitizers, polymerizationinhibitors, and a low molecular weight dicarboxylic acid or anhydridewith a stripping fluid comprised of a mixture of water and a loweraliphatic alcohol; b. heating the resulting mixture of stripping fluidand stripped photopolymerizable composition to cause phase separation;and c. recovering the organic phase, which comprises saidphotopolymerizable composition of matter.
 2. The process of claim 1wherein said heating is sufficient to convert said dicarboxylic acid oranhydride to a low molecular weight dicarboxylic acid diester.